Early transition metal catalysts for olefin polymerization are well-known in the state of the art and comprise the traditional Ziegler-Natta catalysts based on Group 4 and 5 of the Periodic Table (IUPAC notation) and the newer metallocene catalysts, based on Group 4–6 metals. These catalysts systems have been widely used in single-stage processes for olefin polymerization, as well as in multi-stage processes, the latter providing much greater flexibility in controlling the composition and the properties of the end product compared to the former ones.
Multi-stage processes are generally carried out by using the same catalyst in the various stages/reactors, by utilizing tandem reactors operated in series: the product obtained in one reactor is discharged and sent directly to the next stage/reactor, without altering the nature of the catalyst. For example, broad or multi-modal molecular weight distribution (MWD) polyethylenes are commonly prepared by employing a process, wherein ethylene is polymerized in various reactors containing the same catalyst but in the presence of different concentrations of molecular weight regulators.
Multi-stage processes are also used in the preparation of high-impact copolymers of propylene, by sequential polymerization of propylene and of mixtures of propylene with ethylene. For instance, U.S. Pat. No. 4,521,566 describes the preparation of high impact strength polypropylene compositions in a multi-stage process which comprises at least one stage of homopolymerization of propylene and at least one stage of polymerization of ethylene/propylene mixtures in the presence, in both stages, of a catalyst comprising a compound of titanium supported on a magnesium halide.
The international patent application WO 96/02583 describes a multi-stage process wherein different catalyst systems are used in the various stages, in order to allow the obtainment of a wide range of olefinic polymer compositions. More specifically, while in the first stage of the polymerization is carried out in the presence of a Ziegler-Natta catalyst system, in the second polymerization stage a metallocene/alumoxane catalyst system is used, comprising a compound of a transition metal M selected from Ti, V, Zr and Hf, containing at least one M-π bond, and alkyl-Al compound.
The international patent application WO 96/11218 describes a similar process, with the difference that the first polymerization stage is followed by an intermediate stage wherein the catalyst used in the first stage is deactivated, before contacting the obtained polymer with the metallocene/alumoxane system of the second polymerization stage.
In the last years a new family of catalysts for olefin polymerization, based on late transition metals, have been developed in the art; the new catalysts, containing complexes of metals belonging to Groups 8–11 of the Periodic Table of the Elements (new IUPAC notation), exhibit characteristics different from those of transition metal metallocene catalysts or traditional Ziegler-Natta catalysts when used in olefin polymerization.
L. K. Johnson et al. (J. Am. Chem. Soc., 117: 6414–6415, 1995 and J. Am. Chem. Soc., 118: 267–268, 1996) describe the use of Ni and Pd complexes with bidentate α-diimine ligands, useful as catalyst components in ethylene, propylene or 1-hexene polymerization; said complexes are activated with H+(OEt2)2[B(3,5-(CF3)2C6H3)4]−, methylalumoxane (MAO) or Et1AlCl. These systems have the ability to produce highly branched polymers for ethylene and to copolymerize ethylene with polar monomers.
A class of late transition metal complexes of bidentate α-diimine or β-diimine ligands was disclosed in WO 96/23010; said complexes, activated with halo-aluminum alkyl derivatives, MAO or alkylboronic acid derivatives, are used in the oligomerization and polymerization of α-olefins, and in particular of ethylene, and in the copolymerization of ethylene with polar monomers.
Bidentate ligands, which are useful in the preparation of Ni complexes active in the polymerization of ethylene, norbornenes and styrenes, are described in the international patent application WO 97/02298; the corresponding complexes are used in association with acids of a non coordinating monoanion of formula HX, wherein the anion X is preferably BF4-, PF6-, BAF (i.e. tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) or SbR6-.
WO 98/40374 describes olefin polymerization catalysts containing Group 8–10 metals and bidentate ligands having the following formula: wherein the substituents R can be hydrocarbyl, substituted hydrocarbyl or silyl; A and B are heteroatom connected monoradicals, wherein the connected heteroatom is selected from Group 15 or 16, and A and B may be linked by a bridging group; these catalysts optionally contain a Brφnsted or Lewis acid as cocatalyst.
Recently, Brooke L. Small et al. (J. Am. Chem. Soc. 120:4049–4050, 1998) disclosed Fe(II) and Co(II) catalyst systems incorporating tridentate pyridine diimine ligands of the following general structure: wherein R is H, methyl or iso-propyl. The active catalysts, generated by the addition of MAO, are able to convert ethylene to linear high density polyethylene; increasing the steric bulk of the ortho aryl substituents increases molecular weight.
The polymerization of ethylene and propylene with the above-mentioned complexes of pyridine bisimine, and more specifically of 2,6-pyridinecarboxaldehyde bis(imines) and 2,6-diacylpyridine bis(imines), is described in WO 98/27124 and WO 98/30612, wherein the above catalysts are activated with MAO, boron compounds and aluminum alkyl compounds.
The above-described late transition metal catalyst systems have also been used in association with conventional Ziegler-Natta catalyst. International patent applications WO 97/38024 describes polymerization catalyst systems comprising (a) a cyclopentadienyl derivative of a Group 4 transition metal or a catalyst comprising magnesium, titanium and halogen; and (b) a complex of a bidentate ligand with a group 8–10 transition metal compound; in association with suitable cocatalysts. By using at the same time a mixture of the two different catalysts, olefin polymers having a broad molecular weight distribution and excellent formability are obtained.
Moreover, WO 97/48735 concerns a mixed olefin polymerization catalyst system comprising:                one late transition metal catalyst system, consisting of a Group 9–11 metal complex stabilized by a bidentate ligand; and        at least one different catalyst system, selected from a Group 4 metallocene catalyst system and a Ziegler-Natta catalyst system.        
Because late transition metal catalysts may respond differently to reactor conditions than either metallocene or traditional Ziegler-Natta catalysts, by selecting the proper reactor conditions, polymer blends having the desired molecular weight distribution and composition distribution can be obtained. Also in this case, both the catalytic systems are used together in a single reactor.
Finally, the above-mentioned late transition metal catalysts were also supported on inorganic carriers, such as SiO2 and Al2O3, and the obtained supported systems were used in olefin polymerization. For instance, WO 97/48736 describes late transition metal catalyst systems comprising a Group 9–11 metal complex immobilized on a solid metal or metalloid oxide particle support, particularly silica, and their use in heterogeneous polymerization processes.
Catalysts based on late transition metal complexes supported on fine particle carriers consisting of inorganic oxides, such as Al2O3 and SiO2, for use in suspension and gas-phase polymerization processes, were described in JP 09-278821 and JP 09-278822 too; polymers having narrow composition distributions and superior particle performances are obtained.
Nevertheless, the supportation on silica and alumina of late transition metal catalyst leads to lower catalytic activities in comparison with homogeneous polymerization reactions; moreover, the supportation causes a substantial decrease of the branch-producing tendencies (branches/1000 carbon atoms) of these catalysts, thus leading to polymers having greater melting points and lower processability; the branching decrease is estimated around 10–15% in the above-mentioned WO 97/48736.
Therefore, it is felt the need of increasing the catalytic activity of late transition metal complexes with respect to the activities exerted in homogeneous polymerizations and in heterogeneous systems wherein silica or alumina catalysts are used, at the same time preserving the branching-tendency of these catalysts.